The invention relates to a method for immobilizing radioactive wastes for long term storage. More specifically, the invention relates to a method for recovering cesium from solutions containing cesium together with other metal values and for immobilizing radioactive cesium in a highly stable, nonvolatile and insoluble product suitable for long-term storage.
The principal long-term problem caused by nuclear reactor accidents is contamination of the environment with radioactive material as was evidenced by the Chernobyl nuclear reactor accident in 1986. Cesium is a particular problem in this respect because it is very volatile and can be carried in the upper atmosphere for long distances. Therefore, suitable methods of decontaminating the environment, i.e. water and soil, of cesium before its ingestion by animals or humans are highly desirable. Also desirable are methods for recovering radioactive cesium from the gastrointestinal tracts of contaminated animals.
Certain cation exchange resins and various cation exchangers are available which are selective for the recovery of cesium from solution. These include clay minerals and zeolites, both naturally occurring and synthetic. Furthermore, naturally occuring mordenite zeolite has been mixed with animal feed to remove ingested cesium-137 from the gastrointestinal tracts of contaminated animals. Other natural zeolites include clinoptilolite, erionite and chabazite.
In addition to recovering the ions, the radioactive ions must be immobilized in a form suitable for longterm storage so that they cannot be released back into the environment or leached from the storage medium into the surrounding soil or groundwater.
Many methods and materials have been developed over the years for immobilizing various nuclear wastes, and especially cesium, for long-term storage. For example, U.S. Pat. No. 3,161,601 dated Dec. 19, 1964, and assigned to the common assignee, incorporated the radioactive cesium into a glass containing cesium oxide, alumina, phosphate and an additive such as lanthanum or zirconium. Another method was to incorporate the cesium-137 as cesium carbonate with spodumene or with a mixture of silica and kaolin at a temperature of at least 1000.degree. C. to form a synthetic pollucite. Another process mixed an inorganic zeolite containing radioactive cesium with about 20% additives, mainly iron and calcium oxides, which was melted at 1500.degree. C. and control cooled to form an iron-enriched basalt. U.S. Pat. No. 4,537,710, also assigned to the common assignee, describes a cation exchanger which is a modified tobermorite containing aluminum, that is selective for a small number of ions including cesium. The advantage of the modified tobermorite is that it is compatable with matrix materials such as concrete and, therefore, more resistant to leaching.
Other, less satisfactory, methods involve incorporating the radioactive waste directly into a matrix material such as asphalt or concrete before emplacement for storage.
Almost all of the materials or processes suitable or selective for the separation and recovery of cesium from contaminated water or radioactive waste streams require additional processing to immobilize the cesium, in order to prevent the radioactive ions from being leached or otherwise separated from the storage medium. For example, the cesium ions may be eluted from the ion exchanger and incorporated directly into the matrix material for storage. Preferably, the recovery material (ion exchanger) containing the radioactive ions, is itself incorporated into the storage medium, such as a glass or cement, for storage. Alternatively, the radioactive ions may be mixed with other inorganic materials and by applying heat and/or pressure, formed into a synthetic mineral which is satisfactory for storage.
Thus, most processes require several steps to recover and prepare the cesium for storage. This increases the cost of preparing the radioactive ions for storage. Furthermore, some processes require high pressures and/or temperatures which in addition to increasing costs, increases the opportunities for the loss of radioactive material. Finally, many of the storage materials are not wholly suitable for long-term storage because leaching of the radioactive ions can occur.